We know this happens in mice, rats, crickets, caterpillars, and ants — but could it happen in humans? That is, could some of our actions and the choices we make be influenced, at least to some degree, by a parasite or bacteria inside of us? (This thought has been raised before, for example by Carl Zimmer and on Andrew Sullivan’sblog, but I want to explore the issue in more depth here.)

We might come to be influenced by these foreign sources in at least two ways. First, the bacteria, parasite, etc., might have evolved to make us take certain actions that benefit it, even at our expense. A virus that is sexually-transmitted can make us more promiscuous; a bacteria that requires certain nutrients to survive or reproduce can make us crave certain foods. Would you be surprised if doctors later discovered that pica — a disorder where one has an appetite for non-nutritive substances (such as dirt or paper) — is caused by a parasite or infection?

Second, our behavior could be affected as a coincidental byproduct of the foreign source. Like us, bacteria and parasites produce waste; sometimes, that waste is toxic or harmful. It is possible, for example, that the byproduct of some bacterial infection could affect the balance of hormones that drive hunger and make you crave bananas more than usual, even though that has no evolutionary benefit to the bacteria and developed coincidentally. Indeed, some researchers believe that the pesky T. gondii parasite might cause schizophrenia in humans — although not impossible, something tells me that schizophrenic behavior does not benefit the parasite.

There are several implications to this thought. For one, if our decisions could be influenced to some degree by outside agents, then possibly ridding ourselves of that bacteria or parasite can change our behavior. Especially if the behavior is harmful to us and beneficial to the parasite — say, for example, a parasite that makes us overeat — then curing ourselves of the parasite would be beneficial.

Second, the theory could help explain sharp changes in human behavior throughout history that are not best explained by evolution (if there are any such mysteries). That is, if anthropologists observe that humans drastically changed their behavior at a certain point in history, it could be because our behavior evolved, or alternatively because we became infected with an agent that changed our behavior.

Third, it might add an extra layer to the discussion of what actions that we take are truly our choice. If you are infected with a sexually-transmitted parasite that makes you promiscuous, is that the same as you choosing to be promiscuous without the parasite? Or, in the context of criminal behavior — if a bacteria makes you more violent, is that as much your fault — and are you as culpable — as one who chooses to be violent without an infection?

Finally, and I think most interestingly, one important distinction between humans and animals is our capacity to suppress our natural instincts and cravings, and instead make reasoned decisions. Can we rid ourselves of the effects of certain bacteria or parasites through free will alone?

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